The reference circuit is an indispensable part of analog circuits. Other modules of the analog circuit will have an accurate reference point according to the voltage reference point generated by the reference circuit. In fact, as a standard reference point, the reference circuit will work continuously while other analog circuits operate, so the improvement of temperature characteristic and the reduction of power consumption are the eternal topics in the field of reference circuit. In addition, a high power supply rejection ratio and a low operating voltage are also the development directions of the reference circuits.
The reference circuits are divided into two categories depending on whether the resistor is used or not. In general, the reference circuit having resistors has good temperature characteristic, but will occupy a large area of the chip layout, especially in the field of ultra low power reference circuit. If a reference circuit has nano-watt-level power; a resistor of hundreds of mega ohms is required. As a result, the circuit would occupy a large layout area. Therefore, the resistor-less reference circuit is in trend for the low-power reference circuits. However, without the continuous adjustability of the resistors, the temperature characteristic of the -resistor-less reference circuit is generally worse than that of the reference circuit having resistors. Generally, transistors in commonly used reference circuits operate in the saturation region with large current and power. Such a large power is unacceptable in some portable smart medical devices and energy harvesting systems. In order to reduce the power, the application of sub-threshold MOS field-effect transistors in reference circuits is in consideration. However after the sub-threshold MOS field-effect transistors are used, it is difficult to modify the voltage characteristics of the reference circuits, which is also a research direction for low-voltage low-power reference circuits.